The present invention relates to the separation of mixtures using polymer membranes.
Polymer membranes have been proposed for various separations. It has been found that different molecules can be made to diffuse through selected polymers differently. For example if one component of a mixture is found to diffuse through a polymer rapidly and a second component is found to diffuse through the polymer very slowly or not at all, the polymer may be utilized to separate the two components. Polymer membranes potentially can be used for gas separations as well as liquid separations.
Polymeric membrane materials have been found to be of use in gas separations. Numerous research articles and patents describe polymeric membrane materials (e.g., polyimides, polysulfones, polycarbonates, polyethers, polyamides, polyarylates, polypyrrolones, etc.) with desirable gas separation properties, particularly for use in oxygen/nitrogen separation (See, for example, Koros et al., J. Membrane Sci., 83, 1-80 (1993), the contents of which are hereby incorporated by reference, for background and review).
The polymeric membrane materials are typically used in processes in which a feed gas mixture contacts the upstream side of the membrane, resulting in a permeate mixture on the downstream side of the membrane with a greater concentration of one of the components than the composition of the original feed gas mixture. A pressure differential is maintained between the upstream and downstream sides, providing the driving force for permeation. The downstream side can be maintained as a vacuum, or at any pressure below the upstream pressure.
The membrane performance is characterized by the flux of a gas component across the membrane. This flux can be expressed as a quantity called the permeability (P), which is a pressure- and thickness-normalized flux of a given component. The separation of a gas mixture is achieved by a membrane material that permits a faster permeation rate for one component (i.e., higher permeability) over that of another component. The efficiency of the membrane in enriching a component over another component in the permeate stream can be expressed as a quantity called selectivity. Selectivity can be defined as the ratio of the permeabilities of the gas components across the membrane (i.e., PA/PB, where A and B are the two components). A membrane""s permeability and selectivity are material properties of the membrane material itself, and thus these properties are ideally constant with feed pressure, flow rate and other process conditions. However, permeability and selectivity are both temperature-dependent. It is desired to develop membrane materials with a high selectivity (efficiency) for the desired component, while maintaining a high permeability (productivity) for the desired component.
The relative ability of a membrane to achieve the desired separation is referred to as the separation factor or selectivity for the given mixture. There are however several other obstacles to use of a particular polymer to achieve a particular separation under any sort of large scale or commercial conditions. One such obstacle is permeation rate. One of the components to be separated must have a sufficiently high permeation rate at the preferred conditions or else extraordinarily large membrane surface areas are required to allow separation of large amounts of material. Another problem that can occur is that at conditions where the permeability is sufficient, such as at elevated temperatures or pressures, the selectivity for the desired separation can be lost or reduced. Another problem that often occurs is that over time the permeation rate and/or selectivity is reduced to unacceptable levels. This can occur for several reasons. One reason is that impurities present in the mixture can over time clog the pores, if present, or interstitial spaces in the polymer. Another problem that can occur is that one or more components of the mixture can alter the form or structure of the polymer membrane over time thus changing its permeability and/or selectivity. One specific way this can happen is if one or more components of the mixture causes plasticization of the polymer membrane. Plasticization occurs when one or more of the components of the mixture acts as a solvent in the polymer often causing it to swell and lose its membrane properties. It has been found that polymers such as polyimides which have particularly good separation factors for separation of mixtures comprising carbon dioxide and methane are prone to plasticization over time thus resulting in decreasing performance of the membranes made from the polyimides.
The present invention overcomes some of the problems of the prior art membranes by providing a polymer membrane and a method of making said polymer membrane that has the following properties/advantages:
a) Excellent selectivity and permeability,
b) Sustained selectivity over time by resistance to plasticization, and
c) Very large useable surface area by use of hollow fibers.
As discussed above the present invention seeks to provide a membrane and method of making the membrane that achieves the result of providing a commercially viable polymer membrane that overcomes some of the drawbacks of the prior art membranes. The membranes of the present invention can have very large available surface areas using hollow fiber technology. The membranes of the present invention also have a very high selectivity at a very high permeability. The membranes of the present invention also are quite resistant to plasticization and maintain their selectivity and permeability properties over time as is required in commercial applications of this technology. The membrane of the present invention achieves this result by providing a predetermined number of crosslinkable sites in the polymer chain and by crosslinking the polymer membrane using selected crosslinkers. The crosslinkable polymer and crosslinked polymer is achieved without degradation of the imide function (i.e. without altering the polyimide structure.)
In one embodiment of the present invention a hollow fiber mixed matrix polymer membrane is provided, comprising; a crosslinked polymer continuous phase, and a molecular sieve material dispersed within the polymer continuous phase. The resulting membrane can have a CO2 permeability of at least 20 GPU and a CO2/CH4 selectivity of greater than 20, when measured at 35 degrees C. and a pressure of 100 psia.
The productivity (permeance) of a gas separation membrane is measured in GPUs which is defined as follows:   GPU  =                    10                  -          6                    xc3x97                        cm          3                ⁡                  (          STP          )                                    cm        2            xc3x97              sec        .            xc3x97              (                  cm          .                      xe2x80x83                    ⁢          Hg                )            
In an alternative embodiment of the present invention a hollow fiber mixed matrix polymer membrane is provided, comprising: a crosslinked polyimide polymer continuous phase and a molecular sieve material dispersed within the polymer continuous phase having a ratio of crosslinkable sites to imide groups of between 3:8 and 1:16. It has been found that too much crosslinking can cause the hollow fiber polymer to be fragile and can also experience performance problems. Too little crosslinking can lead to plasticization of the polymer membrane over time resulting in deteriorating performance and loss of selectivity.
In another alternative embodiment of the present invention a hollow fiber mixed matrix polymer membrane is provided, comprising a molecular sieve dispersed within a continuous polymer phase wherein said continuous polymer phase comprises a polyimide polymer made from the monomers A+B+C; where A is the dianhydride of the formula; 
where
X1 and X2 are the same or different halogenated alkyl group, phenyl or halogen;
where R1, R2, R3, R4, R5, and R6 are H, alkyl, or halogen;
where B is a diamino cyclic compound without a carboxylic acid functionality;
where C is a diamino cyclic compound with a carboxylic acid functionality; and
wherein the ratio of B to C is between 1:4 and 8:1.
A particularly preferred embodiment of the present invention relates to using the crosslinked mixed matrix hollow fiber polymer membrane of the present invention for the separation of carbon dioxide (CO2) from methane (CH4). In particular this embodiment of the invention relates to the removal of CO2 from natural gas comprising CO2, CH4, and other light gases.
Among other factors the present invention provides the composition of and the method of making a highly effective polymeric membrane for the separation of mixtures. The invention utilizes crosslinking of the mixed matrix polymer membrane to help achieve the high selectivity required to make the separation efficiently and to maintain the high selectivities and other properties even after being exposed to extreme conditions such as high temperatures and pressures. The invention also shows that plasticization of the mixed matrix polymer membrane can be avoided by appropriate degrees of crosslinking and appropriate selection of the crosslinking units. It has also been determined that too much crosslinking can lead to hollow fibers that are brittle and subject to failure. Another feature of the present invention is that the selection of polymer having a proper molecular weight (MW) can be important in the formation of a hollow fiber membrane. It can be preferable to have a MW above the entanglement molecular weight of the polymer. It has been found that if the molecular weight of the polymer is too low the polymer is too brittle and a proper skin layer may not form. If the molecular weight is too high processability can become difficult. It is preferable to have an average molecular weight of between 20,000 and 200,000. The present invention has thus achieved a hollow fiber polymer membrane that is both highly selective and highly permeable for the preferred separations while also being stable and durable for long term use in a commercial separation process at actual working conditions. The present invention also provides a method of making a mixed matrix hollow fiber polymer membrane precursor material that is not excessively fragile allowing effective spinning.
A preferred method for preparing hollow fibers is to dissolve the polymer in a solvent or melt the polymer, and extrude the polymer through a tubular capillary nozzle with a core fluid used for the purpose of retaining the hollow fiber shape.
Any mixture of fluids that differ in size or other properties, for example nitrogen and oxygen or ethylene and ethane, may be separated using the membranes described herein. In one embodiment, a gaseous mixture containing methane and carbon dioxide can be enriched in methane by a gas-phase process through the membrane. In other embodiments, the membranes can be used to purify helium, hydrogen, hydrogen sulfide, oxygen and/or nitrogen.